1. Field of the Invention
The present invention relates to a torque sensor attached on a shaft, more specifically, to a torque sensor for detecting a torque applied on the shaft using magnetostrictive properties.
2. Description of the Prior Art
Many torque sensors are either a magnetostriction type, which uses the changes of magnetic property caused by strain, or an inductance type, which uses the changes of magnetic property caused by stress. As shown, for example, in Japanese Unexamined Patent Publication No. 5-118938, 59-77326, these conventional sensors have two kinds of coils for detecting a torque applied on the shaft, that is, an excitation coil and a detection coil.
The conventional torque sensors have, for example, a construction shown in FIG. 8. A shaft 53 is rotatably supported in a housing 51 by bearings 52. A magnetostrictive member 54 for sensing torque is attached on the periphery of the shaft 53. A cylinder 55 is arranged inside the housing 51 and spaced apart by a predetermined distance from the periphery of the magnetostrictive member 54. Excitation coils 56 and detection coils 57 are fitted in grooves on the inner surface of the cylinder 55. When an alternating current flows in the excitation coil 56, the coil 56 generates a magnetic flux. The flux forms a magnetic circuit passing through the cylinder 55 and the magnetostrictive member 54. The magnetic circuit induces an electromotive force in the detection coil 57.
The magnetostrictive member 54 has two portions divided axially along the shaft 53. Grooves 54a inclined forty-five degrees with respect to the axis of the shaft 53 are formed on one portion, and grooves 54a inclined minus forty-five degrees are formed on the other portion. When a torque is applied to the shaft 53, a tensile force acts on one portion and a compressive force acts on the other portion, straining each portion slightly. Permeability in each portion varies according to the strain. The changes of permeability change the flux passing through the magnetostrictive member 54. Electromotive force is induced in each detection coil 57 according to the changes of flux in each portion of the magnetostrictive member 54.
FIG. 9 shows an electric circuit of conventional torque sensors. The excitation coils 56 are connected to an alternating current power source 58. The detection coils 57 are connected to a controller 59. The controller 59 includes a processing circuit 60, an A/D converter 61, and a microcomputer 62. The processing circuit 60 includes a differential circuit and a rectifier circuit. The microcomputer includes a CPU 63. A signal based on the electromotive force induced by each detection coil 57 is used in the differential circuit of the processing circuit 60 for subtraction. A signal obtained by the subtraction is rectified in the rectifier circuit and input in the CPU 63 through the A/D converter 61. The CPU 63 calculates torque applied on the shaft using the input signal. In this way, conventional torque sensors have a complicated construction requiring the excitation coil 56 and the detection coil 57.